1. Field of the Invention
This invention relates to a two-element group lens capable of correcting leaning of a deflecting plane, and more particularly, to a two-element f.theta. lens group including a cylindrical lens in a scanner optics.
2. Description of the Prior Art
When an optical scanner utilizes light deflecting unit in the form of a rotating multi-facet mirror which causes leaning of a deflecting plane, a non-uniformity in the pitch of scan lines which is caused by the leaning is corrected for by the use of imaging optics which establish an optically conjugate relationship between a deflecting point and a surface being scanned within a plane which is perpendicular to the scan lines.
Such optics exhibit different optical characteristics between the scan direction and a direction perpendicular to thereto, and accordingly, it is a common practice to utilize a cylindrical lens or toric lens. To provide a compact construction for such lens system, it is desirable to provide a single lens arrangement. At this end, a combination of a pair of single lenses is known which produces directivity in the focusing action. However, in an arrangement utilizing a pair of cylindrical single lenses having generatrices which are orthogonal to each other (see Japanese Laid-Open Patent Application No. 015,131/1980), for example, extreme difficulty is experienced in providing a cylindrical lens, in fact two of them, which is known to be difficult to machine as compared with a spherical lens and which is required to exhibit the so-called f.theta. characteristic which allows spotwise scanning of a record surface at substantially a uniform rate. On the other hand, an arrangement utilizing a combination of a spherical single lens and a toric lens (Japanese Laid-Open Patent Application No. 036,622/1981) presents difficulty in maching the toric surface. The f.theta. characteristic refers to the capability to produce a displacement at a substantially uniform rate on a surface being scanned of a spot as a beam is deflected at a uniform angular rate over the surface. The f.theta. lens of the invention provides such capability exclusively with a meniscus single lens.